A source close to the investigation confirmed the theft of diplomatic documents but told news agency AFP: "For the moment, we have no details about the nature of these documents. They could be sensitive documents but they could equally well be unimportant."

The Saudi embassy in Paris could not be reached for comment.

Romain Nadal, a spokesman for the French foreign ministry, said an investigation is underway into the "unacceptable attack".

No suspects have yet been apprehended. Nobody was injured during the attack.

"It's quite an unusual attack. They were obviously well-informed. It's true that it's quite a rare way of operating," one police source said.

Targeting documents "changes nature of crime"

The head of a national police union, Nicolas Comte, said: "We need to find out what they were looking for, the money or the documents."

The investigation has been turned over to the Banditry Repression Brigade (BRB) special police unit in charge of armed robberies.

One source close to the investigation, who did not wish to be named, said: "If they were looking for sensitive documents, that changes the nature of the crime."

"It will no longer be an armed robbery, but something more complicated," the source said.

The source added that the initial results of the investigation revealed the robbers were obviously "aware of what they would find by attacking that specific car and not the others".

"We're dealing with a heavily armed and determined group of criminals," said Frederic Lagache, from the Alliance police union.

"Once again, unfortunately, we see that these individuals had no fear of the police and were not concerned about the consequences of their actions."